Variable attenuator for centimeter waves



P 1947- I E. o. KEIZER. 2,427,098

VARIABLE ATTENUATOR FOR CENTIMETER WAVES Filed 0ct. -23, 1943 ZSheets-Sheet '1 inventor .aQ AM Gttorneg Sept. 9, 1947.

' E. o. KEIZER VARIABLE ATTENUATOR FOR CENTIMETER WAVES Filed Oct; 23, 1945 2 Sheetg-Sheef 2- Zinnentor ZZ/MMJK lzm Gttorneg Patented Sept. 9, 1947 UNITED STATES PATENT OFFICE. VARIABLE ATTENI iiiZZSFQR CENIiIMETEB- Eugene 0. -Kcizer, l%rinccton, N2 Jo, assignor to Radio Corporation of America, a corporation of Delaware Applicationflctober 23, 1943, Serial'Nm 50 7 467 This invention 7 relates generally to :microwave apparatus and more particularly to improved methods of and means for providing variable attenuation of microwaves in a waveguide transmission system.

It is well known in the art that considerable attenuation of micro-waves is provided byconstrictingthe wallsof amiorowave waveguide to a dimension less than one-half wavelength along the magnetic axis of wave propagation when the dielectric within the waveguide is air. For high dielectric constant materials the cut-off wavelength-is increased in proportion to the value of the dielectric constant; The attenuation provided-by such constricted waveguidesincreases rapidly as the transmitted wavelength is increased above-thecritical value. The instant 9 Claims. (Cl. 1781-44) invention utilizes this property of waveguide transmission systems -by-- employing a section of waveguide-having tapered portions for matchingmissionapath having low attenuation, since the efiective-cutmff wavelength of this portion of theguidewill beabove that of the transmitted energy. The resultant attenuation of the constricted waveguide portion maybe dependent uponthe volume of thehighdielectric constant 'material' which is inserted within the constricted portion of the waveguide.

One form of variable 'attenuatorproviding sat isfactoryattenuation adjustment comprises a waveguide having: a constricted-portion which en closesa tapered slug-of high'dielectrlc constant solid material which may-jbe moved longitudinally along the guidebymeans of projections whichextend athrough: longitudinal slots at in 1 the guide faces. This embodiment of the invention may employ circular, elliptical "or rectangular Waveguides having all or predeterminedopposite faces of-theguide constricted,- asdesired.

Another embodiment of the invention which may be employed comprises aconstricted "waveguide portion :which includes means for introduoing: therein a high dielectric constant z liquid.

The .liquid. may 'be introduced by any convenient hydraulicmeans-to a-desired depth within a lim ited length ofth'econstricted-waveguide portion;

or;theliquidmaytbe:inserted to provide-a variable length liquid dielectric path withina U-shaped portion of; the constricted waveguide.

Among the obj cots- 0f 'Ihe invention are to provide an improved method: of and-meansfor attenuating microwave energy within a waveguide transmission systemr Anotherobject of the, in-

ventionisizto, provide animproved imethod of and means ;for; attenuating.- microwave energywithin a waveguide transmission system by providing an adjustable length ofconstricted:waveguide having cut-ofi z frequency higherthan the frequency of the; energy to-ybetransmittedl An additional object of the invention'istto provide. an improved method; of W and means yfor attenuating microwave rial..may be adjust'ablyintroduced: into; a constricted portion of the waveguide; transmission system whereby efiective microwayentransmission is accomplished through the, high 1 dielectric constant material while: desired attenuationis pro.- vided .by the constrictedzwaveguide; portion 1 em: ployinesalow'.contsant dielectric;v Another ob.- ject ,of;the;inventi,0n1 is to provide an. improved method; of: and means for providing P adjustable attenuation :of microwave; energy in ,a waveguide transmission I system wherein "a. high dielectric constant 1lowelossliquidzisxintroduced.into a portion of;a constricted waveguide.- section tolower the rcuteofi Wavelength :of said constricted :guide portion, An additional objectjot the inventionis to providean improvedjadjustableattenuator for microwave energy, havingminimum wave. reflection c aracteristics due rte-discontinuities there- Thesinvention willsbexfurther :describedibylref erence: to the accompanying drawings .of stwhich Figure, 1 is aIIJBISPBCtiYBIViGW ofcone embodiment thereof, Figure .2xis; across-sectional view: of. Fig- .ure lftaken .alonguthe. section line II-.II, Figure 3 is a perspective-view of a modification of thedevice shown lin Figures land 2, Figure 4 .is' a.

perspective view. ofaa' second .iembodiment of ithe invention,-, Figure. .5 is .a. fragmentary cross-{sectional :viewv of :Figure 4 2 taken along the section line ;V- V, and Figure fi-isea perspective: view: of

a thirdxem-bodiment; of .the invention. Similar reference characterszare applied.- to, similainelements throughout the drawing.

ReferringstoFigures 1 and, 2;.a waveguide J of rectangular cross-section ,.includes a' constricted portion :3 5. coupled thereto. by t means of tapered portions .15.; The narrowa-side: Walls .of: :the con-- strict-edportion .3:includelongitudinal. slotsfl. A slug 9,5. of low-loss imaterial ihaving anrelatively highdielectric constant, is inserted withinthe constricted waveguide portion =3,'and proportioned l to have "substantially thesame cross-sectional dimensions as theconstrictedguide. The ends'll of the high dielectric constant slug may" be-taenergy: in, a; waveguide: transmission system, whereinaa low-loss. high dielectric constant matepered to minimize wave reflections from the ends of the slug in both directions along the main waveguide I. Adjusting elements such as, for example, pins I3 secured to the narrow sides of the slug 9, extend outwardly through the slots 1 in the narrow side walls of the constricted waveguide portion 3. The pins I3 may b actuated in any convenient manner to adjust the longitudinal position of the slug 9 within the constricted waveguide portion 3. Another type of adjusting element, providing convenient operation, comprises a continuous cable drive 29 secured to each end of the slug and brought out through suitable guide apertures 3I in the waveguide walls to a conventional dial cable drive which includes guide pulleys 33, drive pulley 35 and an indicator 37.

As explained heretofore, wave transmission within the waveguide portions I will be subjected to extremely low attenuation since the cutoff frequency'of this portion of the guide is below that of the microwave energy to be transmitted therein. The cut-off frequency of the constricted portion 3 of the waveguide is higher than that of the transmitted energy when the constricted guide portion includes an air dielectric. However, when the high dielectric constant material 9 occupies the constricted waveguide portion 3, the cut-off frequency of the constricted guide portion is lowered to a value below the frequency of the transmitted microwave energy. The resultant attenuation of the constricted waveguide portion 3 therefore will be proportional to the relative lengths of the constricted waveguide portion having an air dielectric and the portion of the constricted waveguide having a high dielectric constant material therein. It will be seen that the attenuation of the constricted waveguide portion may be varied from a very low value when the entire constricted waveguide includes high dielectric constant material to a relatively high value when the high dielectric constant material is substantially removed from the constricted waveguide portion. The maximum attenuation obtainable will depend upon the cross-sectional and longitudinal dimensions of the constricted waveguide. For example, an attenuation of 10 db. per centimeter along the longitudinal waveguide axis is obtainable in a constricted waveguide having a length along the magnetic axis of the order of eight-tenths the cut-off length at an operating wavelength of 3 cm. The attenuation may be varied readily from 5 to 20 db./cm. along the longitudinal axis by suitably proportioning the crosssectional dimensions of the constricted waveguide portion. Another example indicates that an attenuation of db./cm. may be obtained at wavelengths of 10 cm., when the length along the magnetic axis is six-tenths the cut-off length.

Figure 3 is a modification of the device described in Figure 1 wherein only the narrow side faces 4, defining the limits of the magnetic axis of the waveguide, are constricted. The pins I3 extend through slots 8 in the oppositely disposed wide faces of the constricted portion 3 of the waveguide. With this arrangement, only the narrow side faces of the Waveguide I will be tapered to the constricted narrow faces 4 of the waveguide by means of tapered portions 5. The operation of the devices of Figures 1 and 3 will be substantially identical, since the length of the electrical axis of the waveguide does not affect substantially th cut-off frequency. The modification shown in Figure 3 is preferable mechanically to that disclosed in Figure l for the reason that the upper and lower faces of the high dielectric 7 slots of the device of Fig. 1, since in Fig. 3 the slots are at the region of the strongest electric Referring to Figures 4 and 5, only the narrow side faces 4 of the waveguide I are tapered, as in the device described in Figur 3. The constricted portion 3 of the waveguide includes two inclined partitions 6, 6' of low-loss insulating material which close off the ends of the constricted guide portion to provide a substantially liquid-tight compartment ID. The upper face of the constricted waveguide portion 3 includes a vent I2, while the lower face of the liquid-tight compartment Ill includes an outlet I4 to which is connected one end of a U-shaped liquid reservoir I5. A piston II, actuated by a piston shaft I9 and a control knob 2| which extend outside of the remaining end of the U-shaped reservoir I5, provide hydraulic means for raising or lowering the level 23 of a liquid low-loss dielectric 25 within the liquid-tight chamber I0.

As the control knob 2I is moved toward the end of the U-shaped reservoir I5, the piston I1 forces additional liquid dielectric 25 into the chamber II], which lowers the cut-off frequenc of the constricted waveguide portion 3. When the chamber I3 is substantially filled with liquid dielectric, the attenuation of the constricted waveguide portion 3 reaches a minimum value, while as the level 23 of the liquid dielectric 25 is lowered in the chamber ID the attenuation is increased to a value dependent upon the crosssectional and longitudinal dimensions of the constricted waveguide portion. The insulating partitions 6, 3 may be inclined with respect to the longitudinal axis of the waveguide as much as required to reduce wave reflections in the main waveguide I from the insulating partition walls. The smaller the angle formed between the insulating partitions 6, 6' and the upper and lower waveguide faces, the lower will be the standing wave ratio in the waveguide I.

More uniform variation of attenuation may be obtained if the waveguide and the constricted portion thereof were rotated and the outlet port I4 and vent I2 were located in opposite narrow guide faces, instead of as illustrated in Figure 5. With this arrangement the width of the constricted waveguide would be varied along the magnetic axis instead of along the electric axis as in the device of Fig. 5.,

Figure 6 is another modification of the invention wherein the constricted waveguide portion 3 is U-shape to provide a suitable open-ended chamber for the liquid dielectric. The main waveguide I may be tapered in any manner described heretofore, and the elbows. 27 of the constricted waveguide portion 3 preferably should be Well rounded or shaped to minimize reflections to th main waveguide I. In this embodiment of the invention, the vent I2 in the waveguide face 3 is not required, since venting is obtained directly into the main waveguide I. The aperture I 4 in the lower face of the U-shape constricted waveguide 3 opens into a liquid reservoir I5 having a piston 'I'I disposed therein. The piston I! may be actuated by a control shaft I9 and control knob 2| as described heretofore in Figures 4 and 5. The fluid reservoir I5 may be constructed as a U tube of the type described in Figure 4 or it may be in any other convenient form. As the control knob 2| and control shaft 19 are moved toward the end of the fluid reservoir l5, the piston ll forces the fluid dielectric 25 into both arms of the U-shape constricted waveguide portion 3. It will be seen that as more fluid is forced into the U-shape constricted waveguide portion 3, the length of the constricted waveguide portion which includes a high-constant dielectric is increased with respect to the length of the constricted waveguide portion which includes an air dielectric. Preferably, the fluid should not be permitted to drop below the top of the horizontal portion of the U-shape constricted portion of the guide, thereby minimizing nonlinearity in attenuation adjustment, and simplifying the guiding of the resistive material 29 floated on the liquid surfaces, as described hereinafter.

In order to minimize wave reflections from the surfaces of the fluid 25 within the U-shape constricted waveguide portion 3, suitable resistive material 29 such as, for example, conductive rubber or some moderately high resistive liquid, may be floated upon the surfaces of the low-loss high dielectric constant liquid 25. The impedance of the resistive elements 29 should be calculated to minimize the reflection of microwave energy as it passes between the air-filled and dielectricfilled portions of the constricted waveguide.

Thus the invention describedcomprises several modifications of a device wherein the normally high attenuation of a constricted waveguide portion is adjusted by introducing therein high-dielectric-constant materials which lower the attenuation of the guide portion enclosing said materials.

I claim as my invention:

1. A variable microwave attenuator including a waveguide transmission system having a constricted portion thereof providing a region of relatively high attenuation at the operating microwave frequency and mode of wave propagation with a low dielectric constant dielectric material in said constricted portion, a relatively high-constant liquid dielectric occupying a predetermined portion of said constricted portion,

and means for adjusting the dimensions of said portion and said predetermined portion.

2. A variable microwave attenuator including a waveguide transmission system of rectangular cross-section having a portion constricted along only one of the transverse axes thereof providing a region of relatively high attenuation at the operating microwave frequency and mode of wave propagation with a low dielectric constant dielectric material in said constricted portion, a relatively high-constant dielectric occupying the entire cross-section and a predetermined portion of the longitudinal dimensions of said constricted portion thereby providing a relatively low attenuation region along said predetermined longitudinal portion and a relatively high attenuation region along the remainder of said constricted portion, and means for adjusting the length of said low attenuation region.

3. Apparatus of the type described in claim 2 characterized in that said high-constant dielectric is a liquid.

4. Apparatus of the type described in claim 1 including a reservoir for said liquid, and means for controlling the volume of said liquid transferred from said reservoir to said predetermined portion of said waveguide.

5. A variable microwave attenuator including a waveguide transmission system having a U- shape constricted portion thereof providing a region of relatively high attenuation at the operating microwave frequency and mode of wave propagation with a low constant dielectric material in said constricted portion, a relatively high-constant liquid dielectric occupying a predetermined portion of said constricted portion thereby providing a relatively low attenuation region along said predetermined portion and a relatively high attenuation region along the re mainder of said constricted portion, a reservoir for said liquid dielectric, a port interconnecting said constricted portion and said reservoir, and

means for controlling the volume of liquid transferred from said reservoir to said constricted portion for adjusting the relative lengths of said high and said low attenuation regions.

6. A variable microwave attenuator including a waveguide transmission system of rectangular cross-section having a portion constricted along only one of the transverse axes thereof providing a region of relatively high attenuation at the operating microwave frequency and mode of wave propagation with a low dielectric constant dielectric material in said constricted portion, a relatively high constant dielectric occupying a predetermined portion of said constricted portion, and means operable externally of said waveguide for adjusting the dimensions of said predetermined portion.

7. A variable microwave attenuator including a waveguide transmission system of rectangular cross section having a portion constricted along only the magnetic transverse axis thereof providing a region of relatively high attenuation at the operating microwave frequency and mode of wave propagation with a low dielectric constant dielectric material in said constricted portion, a block of relatively high-constant dielectric material tapered at both ends to minimize wave reflections therefrom and occupying the entire cross-section and a predetermined portion of the longitudina1 dimensions of said constricted portion thereby providing a relatively low attenuation region along said predetermined longitudinal portion and a relatively high attenuation region along the remainder of said constricted portion, and means for moving said block longitudinally in said constricted waveguide portion for adjusting the relative lengths of said high and said low attenuation regions.

8. Apparatus of the type described in claim 5 including impedance matching means floated upon the surfaces of said liquid dielectric within said constricted waveguide portion for minimizing wave reflections from said surfaces.

9. Apparatus of the type described in claim 1 including tapered confining means enclosing said liquid within said constricted wave-guide portion for minimizing wave reflections from said liquid.

EUGENE O. KEIZER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,197,123 King Apr. 16, 1940 2376,7823 Krasik May 22, 1945 

